The storage of electrical energy with the aid of electrochemical energy storage systems such as electrochemical capacitors (supercapacitors) or electrochemical primary or secondary batteries has been available for many years. These energy storage systems differ in their underlying principle of energy storage.
Supercapacitors generally include a negative electrode and a positive electrode that are separated from one another by a separator. In addition, an ion-conductive electrolyte is situated between the electrodes. The storage of electrical energy is based on the fact that when a voltage is applied to the electrodes of the supercapacitor, an electrochemical double layer forms on the surfaces of the electrodes. This double layer is formed from solvated charge carriers from the electrolyte, which become arranged on the surfaces of the oppositely electrically charged electrodes. In this type of energy storage, a redox reaction is not involved. Theoretically, supercapacitors may therefore be charged as often as desired, and thus have a very long service life. In addition, the power density of the supercapacitors is high, whereas the energy density is rather low compared to lithium-ion batteries, for example.
In contrast, the energy storage in primary and secondary batteries takes place via a redox reaction. These batteries also generally include a negative electrode and a positive electrode that are separated from one another by a separator. An ion-conductive electrolyte is likewise situated between the electrodes. In lithium-ion batteries, one of the most commonly used secondary battery types, the energy storage takes place via the intercalation of lithium ions into the electrode active materials. During operation of the battery cell, i.e., during a discharging operation, electrons flow in an external circuit from the negative electrode to the positive electrode. During a discharging operation, lithium ions migrate from the negative electrode to the positive electrode within the battery cell. In the process, the lithium ions are reversibly deintercalated from the active material of the negative electrode, also referred to as delithiation. During a charging operation of the battery cell, the lithium ions migrate from the positive electrode to the negative electrode. In the process, the lithium ions are reversibly reintercalated into the active material of the negative electrode, also referred to as lithiation.
Lithium-ion batteries are characterized in that they have a high energy density; i.e., they are able to store a large quantity of energy per mass or volume. However, in return they have only a limited power density and service life. This is disadvantageous for many applications, so that lithium-ion batteries cannot be used, or can be used only to a limited extent, in these areas.
Hybrid supercapacitors represent a combination of these technologies, and are suitable for closing the gap between the application options in lithium ion battery technology and supercapacitor technology.
Hybrid supercapacitors generally likewise include two electrodes, which in each case include a current collector and are separated from one another by a separator. The transport of the electrical charges between the electrodes is ensured by electrolytes or electrolyte compositions. As active material, the electrodes generally include a conventional supercapacitor material (also referred to below as statically capacitive active material) and a material that is capable of entering into a redox reaction with the charge carriers of the electrolyte and forming an intercalation compound therefrom (also referred to below as electrochemical redox active material). The energy storage principle of the hybrid supercapacitors is thus based on the formation of an electrochemical double layer in combination with the formation of a Faraday lithium intercalation compound. The energy storage system thus obtained has a high energy density, and at the same time, a high power density and a long service life.
However, the performance of conventional hybrid supercapacitors is often limited by the conductivity of the electrolyte composition. In particular in the area of electromobility, there is a need for energy storage systems that have preferably high performance.
The use of electrolyte additives in electrolyte compositions to improve the properties of lithium-ion batteries is described in the related art, for example in Journal of Power Sources 162 (2006) 1379-1394. Electrolyte additives, which are used to improve the ion solvation in electrolyte compositions of lithium-ion batteries, are described, for example, in Zhang, Journal of Power Sources 162 (2006) 1379-1394; Lee et al., J. Electrochem. Soc. 145 (1998) 2813; Sun et al., J. Electrochem. Soc. 146 (1999) 3655; Zhang et al., J. Electrochem. Soc. 143 (1996) 4047; Angell, U.S. Pat. No. 5,849,432 (1998); Sun et al., J. Electrochem. Soc. 149 (2002) A355; Sun et al., Electrochem. Solid-State Lett. 1 (1998) 239; Sun et al., Electrochem. Solid-State Lett. 4 (2001) A184; Sun et al., Electrochem. Solid-State Lett. 5 (2002) A248; Sun et al., Electrochem. Solid-State Lett. 6 (2003) A43; Lee et al., J. Electrochem. Soc. 149 (2002) A1460; Lee et al., J. Electrochem. Soc. 151 (2004) A1429; Lee et al., J. Electrochem. Soc. 143 (1996) 3825; and Lee et al., J. Electrochem. Soc. 147 (2000) 9.
U.S. Pat. No. 8,081,418 describes a double layer capacitor that includes an electrolyte composition to which an additive for lowering the melting point of the electrolyte composition is added in order to improve the use at low temperatures.
U.S. Pat. No. 8,586,250 describes, among other things, a nonaqueous electrolyte composition for a lithium ion capacitor, including a lithium salt, and a solvent mixture of a hydrofluoro ether of formula CF3CH2OCF2CF2H and a carbonate solvent. The solvent mixture is used for improved solvation of the lithium salt at low temperatures.
An object of the present invention is to provide an electrochemical energy storage system having improved conductivity via an electrolyte composition.